Changed font used in graphviz (.dot) files
Fix error pulling out contigs with population

Fixes issue #5

1. Only use seed kmers that are in the sample we are pulling out contigs for
2. Allow pooling colours when checking sample names match
3. graph_file_reader.c, gpath_reader.c: paths are const char* not char* now
4. db_alignment.c: keep track of kmer origin position or -1 if inferred kmer
5. correct_alignment.c: operates on internal dBAlignment and takes reads as input
6. Print VCF header from JSON input headers
7. gpath_load_sample_pop() now won't complain if you give multi-colour graph file
   and only ask to load a single colour from it.
8. assemble_contigs.c: When pulling out a contig, give end status as HitRepeat
   if GraphWalker ok but RepeatWalker stopped.